Tall oil soap (TOS) is a by-product of the Kraft process. The soap consists mainly of sodium salts of fatty acids, sodium salts of diterpenic (rosin) acids, free fatty acids, free rosin acids and unsaponifiable neutral compounds such as fatty alcohols, sterols, steryl esters and waxes. The water content of neat soap is typically 25 to 45%, such as 30 to 35%. The pH of the soap is typically in the range of 12 to 13, arising from the presence of entrained black liquor. In industrial practice, soap is routinely acidulated with sulphuric acid to produce crude tall oil (CTO), an article of commerce. Sterols, in particular phytosterols, have several uses, including the use as food additives and as precursors for steroids. Several methods have been reported for the isolation of sterols from tall oil soap. The general method involves the extraction of neat soap with a variety of organic solvents. The presence of entrained black liquor promotes and stabilizes the unwanted water-oil emulsion, which is known to be extremely difficult to break. Consequently, the efficiency of solvent extraction of neat soap for the isolation of sterols is greatly reduced. Holmbom et al. teach in U.S. Pat. No. 3,965,085 the extraction of a mixture of acetone-water soap slurry, with a water-immiscible solvent such as hexane. The aqueous phase contains mainly sodium salts of fatty and resin acids. The organic phase contains mostly unsaponifiables including sterols. In U.S. Pat. No. 4,044,031, Johansson et al. teach the dissolution of soap in a water-immiscible mixture comprising hexane and acetone, extraction of the water-immiscible phase with another solvent mixture comprising methanol or acetone and hexane and water, and isolation of sterols from the methanolic phase by evaporative crystallization. In U.S. Pat. No. 5,770,749, Kutney et al. teach the use of a mixture of ketones, hydrocarbons and water to extract sterols from soap. The hydrocarbon extract is further processed with methanol. However, the complexity of recovering the multi- component spent solvent is very problematic in these processes. Also, recycling highly water soluble solvents such as acetone and methanol from water mixtures is very complicated.
Further examples of methods of obtaining phytosterols are described in US2005/0107582, W02009/106696 and CA2319230.
There is thus a need for a more efficient method of obtaining phytosterols from tall oil soap.